Internal bleeding detection apparatus

ABSTRACT

An EIT system  1  adapted to detect internal bleeding in a body portion, the EIT system  1  comprising a plurality of electrodes  3  adapted in use to extend in a substantially linear orientation across one side only of the body portion and to be applied in electrical contact with the skin of the body portion, a current source adapted to cyclically apply an electric current between one pair of the electrodes  3 , a voltage measuring means to measure the voltage across each of the other pairs of the electrodes resulting from the current, a data collection system  2  and a data analysis system  4  to analyze data resulting from the voltages that are measured by the voltage measuring means, wherein the analysis system  4  is configured to obtain quantitative information related to amounts and rates of conductive tissue changes occurring in the body, based on an EIT analysis equivalent to that obtained from data derived from electrodes spaced around the full perimeter of the body portion. Also disclosed is an electrode belt suitable for bioelectrical use and in particular for detection of change of volume of tissue in a body portion.

FIELD OF THE INVENTION

The present invention relates to an apparatus and method for theanalysis of body tissue by Electrical Impedance Tomography. It isparticularly applicable for detecting or monitoring change in volume ofthe body tissue. A particular application of the invention is thedetection of internal bleeding within a living body, particularlyintraperitoneal bleeding. Also disclosed is an electrode belt suitablefor bioelectrical use and in particular for detection of internalbleeding.

BACKGROUND ART

Electrical Impedance Tomography (EIT) is an imaging method that seeks tocreate cross-sectional maps of electrical resistivity or impedancedistribution inside the body. This has previously been done using a 16electrode array fixed to the external perimeter of a body about atransverse plane for example as described in U.S. Pat. No. 4,617,939(Brown & Barber). The electrical current causes a change in theelectrical potential on the surface of the body being examined. Theother electrodes of the array are used to measure the electricalpotential on the surface of the body and thereby monitor the electricfield created by the current pattern. Distortions in the field patternmay be related to changes in the impedance distribution inside the body.As the solution of impedance distribution from surface voltagemeasurements is generally ill-posed, it has not been effective forproducing good static images of body organs. This has limited theadoption of the technique for general use.

The EIT process may be contrasted with other bio-electrical proceduressuch as bioimpedance spectroscopy. Bioimpedance spectroscopy is aprocess whereby four electrodes are situated at standard referencepoints on the body (for example, right and left wrists, right and leftankles). The actual positioning of the electrodes could vary withapplication. The impedance measurements are made with this group offour. Two electrodes are nominated for current flow and the other twoare used for measuring voltages. Impedance is measured as a function offrequency (say, over the range 1 kHz to 1 MHz) and the results may bedisplayed as an R vs. X (resistance vs. reactance) plot over this rangeor simply as the modulus |Z| or phase or some version of this. Theimpedance or R or X or related measure would be considered as adependent variable of measures such as for example % water or % fat,sex, height, extent of bleeding (these being things that may be given apriori or solved for) in a standardized empirical function and so agiven Z would be used to extract a parameter such as the extent ofbleeding. The success of the process depends significantly on how goodthe empirical function is and how ‘standard’ the subject. Use of thisprocedure to detect intraperitoneal bleeding has had very limitedsuccess.

Serious injury to internal organs—for example, as can be suffered duringblunt trauma associated with road accidents—is usually indicated by thepresence of internal bleeding. It is the rate of the internal bleeding,in addition to the total amount of blood lost, which is indicative ofserious injury and relative urgency of treatment. A rate of more than 30ml per minute is usually an indication that intervention may benecessary. Bleeding is usually monitored by monitoring vital signs suchas pulse rate, blood pressure and skin colour. However, this is notalways a consistent way to detect serious internal bleeding—particularlyamong younger trauma victims.

The use of EIT for detecting bleeding was discussed in the paper:“Detection and Quantification of Intraperitoneal Fluid Using ElectricalImpedance Tomography” by Rosalind J Sadleir and Richard A Fox, IEEETransactions on Biomedical Engineering, Vol. 48, No. 4, April 2001,pages 484-491.

While EIT has shown considerable promise for detection ofintraperitoneal bleeding and other uses, its use has been limited due tocertain problems inherent in the technique as used to date. The problemsolved by EIT is inherently non-linear which has limited the usefulnessof images reconstructed according to linearized approximations.Additionally, the accuracy of the results is limited due to extraneousvariations occurring during the test period. Chief amongst these is theeffect of breathing. Impedance measurements are particularly sensitiveto the changes in abdominal shape and lung air quantity during thebreathing cycle. In addition, the electrodes previously used forobtaining EIT images of the abdomen have typically comprised a belt with16 electrodes adapted to be positioned all around the perimeter of theabdomen. This can be problematic for practical use on patients,especially those where spinal injury is involved. Such belts have alsobeen susceptible to pick up of electrical noise on voltage inputs.

Throughout this specification, the term “tissue” will be taken here toinclude fluids such as blood and lymphatic fluids as well as other typesof tissue.

The above description of the prior art is given to assist the readerform an understanding of the nature of the invention disclosed herein.However, this description is not to be taken as indicating that thedisclosure in that prior art in any way forms part of the common generalknowledge in the art.

DISCLOSURE OF THE INVENTION

According to a first aspect, the invention resides in an EIT systemadapted to detect changes in tissue volume within a body portion, theEIT system comprising a plurality of electrodes adapted in use to extendin a substantially linear orientation across one surface only of thebody portion and to be applied in electrical contact with the skin ofthe body portion, a current source adapted to cyclically apply anelectric current between one pair of the electrodes, a voltage measuringmeans to measure the voltage across each of the other pairs of theelectrodes resulting from the current, a data collection system and adata analysis system to analyse data resulting from the voltages thatare measured by the voltage measuring means, wherein the analysis systemis configured to obtain quantitative information related to amounts andrates of conductive tissue changes occurring in the body, based on anEIT analysis equivalent to that obtained from data derived fromelectrodes spaced around the full perimeter of the body portion.

According to a preferred feature of the invention, the processing meansestablishes a model of the body portion under analysis comprising aplurality of elements and wherein a parameter representative of anelectric field present in each element resulting from the current iscalculated from the voltages that are measured and wherein the values ofat least a portion of the parameters that are calculated for theelements are amended to substantially reconstruct values that would beobtained from measurements of voltages around the perimeter of the bodyportion and wherein the change of value of the parameter in a portion ofelements over time is indicative of internal bleeding within the bodyportion.

According to a preferred feature of the invention, the data analysissystem implements a series of steps to reconstruct the parameter valuesof the elements, the steps comprising:

-   -   calculate the difference between a reference data set and a        measured data set of the voltages as measured to establish a        vector;    -   multiply the data set by a reconstruction matrix to obtain a        reconstructed image having a plurality of pixels;    -   integrate the values of the pixels in the reconstructed image to        obtain a value of the parameter;    -   apply spatial filtering to correct for non-uniformity of        parameter over the image plane    -   monitor change in the value of the parameter over a period of        time to provide an indication of change of tissue volume.

According to a preferred feature of the invention, a detected change intissue volume is representative of internal bleeding.

According to a preferred feature of the invention, the parameter isdefined as Resistivity Index calculated in accordance with one of:

${RI} = {{\int_{\Omega}^{\;}{d\;\sigma\ d\; S\mspace{14mu}{or}\mspace{14mu}{RI}}} = {\sum\limits_{p = 1}^{TP}\;{d\;\sigma\; d\; A_{p}}}}$

-   -   for a two-dimensional array, or

${RI} = {{\int_{\Omega}^{\;}{d\;\sigma\ d\; V\mspace{14mu}{or}\mspace{14mu}{RI}}} = {\sum\limits_{p = 1}^{TP}\;{d\;\sigma\; d\; V_{p}}}}$

-   -   for a three-dimensional array, where dA_(p) and dV_(p) are the        areas or volumes of two or three dimensional image elements        respectively.

According to a preferred feature of the invention, the data analysissystem further implements the steps of:

-   -   using empirical sensitivity calibration to provide an estimate        of the parameter in terms of blood volume;    -   dividing the estimated blood volume by time interval between        reference and measured data sets to provide an estimate of the        rate of bleeding;    -   determining an alarm category depending on the rate of bleeding        that has been calculated:

According to a preferred feature of the invention, the data analysissystem applies a digital filter to the data to provide temporalfiltering of the data to thereby remove or at least minimise the effectof breathing on the EIT analysis.

According to a preferred feature of the invention, the electrodes areprovided in a belt adapted to be lain across an anterior surface of thebody portion and having a length such that the ends to not extend fullyaround said body portion, the electrodes being spaced along the lengthof the belt.

According to a preferred feature of the invention, each electrodecomprises a contact face adapted to contact the skin of the body portionwherein the contact face is of elongate form having an elongate axisoriented substantially transverse to the linear spacing of theelectrodes.

According to a preferred feature of the invention, the current source,voltage measuring means and data collection system are associated withan on-patient module adapted to be carried by the body having the bodyportion, wherein the data analysis system is provided by a remoteprocessor and wherein data communication is provided between theon-patient module and the remote processor.

According to a preferred feature of the invention, the datacommunication is by wireless communication.

According to a preferred feature of the invention, the on-patient datamodule comprises processing circuitry and a telemetry transceiver thatwill allow data to be transferred to and from the processor.

According to a preferred feature of the invention, the processingcircuitry selects the pair of electrodes to which a current is appliedand the pair of electrodes across which voltage is measured at any pointin time.

According to a further aspect, the invention resides in a method fordetecting changes in tissue volume using an EIT system, the methodcomprising the steps of:

-   -   applying a current between a pair of electrodes spaced at the        surface of a body portion;    -   measuring, at predetermined intervals, and at a multiplicity of        locations in a plane through the body portion, the voltage        between pairs of electrodes at the surface of the body portion        resulting from the applied current to provide a set of voltage        measurements, wherein the electrodes extend in a substantially        linear orientation across one side only of the body portion;    -   determining the changes of the voltage measurement between        consecutive sets of voltage measurements;    -   generating a reconstructed image of the body portion;    -   determining the resistivity index of the tissue within the body        portion from the reconstructed image;    -   deriving a volume of tissue from the determined resistivity        index;    -   determining the rate of change of tissue volume between        consecutive sets of voltage measurements.

According to a preferred feature of the invention, the method includesthe further step of initiating an alarm where the rate of change oftissue volume is above a predetermined value.

According to a preferred feature of the invention, the resistivity indexis calculated by generating a vector indicative of the changes involtage measurements between consecutive sets of voltage measurements;and multiplying the vector by a reconstruction matrix, the resultantmatrix being the reconstructed image.

According to a preferred feature of the invention, the resistivity indexis calculated by integrating the pixel values from the reconstructedimage.

According to a further aspect the invention resides in an electrode beltadapted for use with an EIT system, the belt comprising a plurality ofelectrodes spaced along the elongate length of the belt and havingcontact faces adapted to make electrical connection with the skin of abody portion under examination wherein the belt is adapted to provideengagement of the electrodes on one side only of the body portion.

According to a preferred embodiment, the contact faces are substantiallyrectangular with a length in the range of 75 mm to 100 mm and a width inthe range of 5 to 25 mm.

According to a preferred feature of the invention, the belt is flexibleto enable the belt to conform to the profile of the body portion uponwhich it is placed to facilitate contact of each electrode with the bodyportion.

According to a preferred embodiment, the body portion is the abdomen andthe ends of belt are formed with a curvature to facilitate contact ofelectrodes in the vicinity of the sides of the abdomen when in use.

According to a preferred embodiment at least some of the electrodes areprovided with an adhesive surround to facilitate secure engagement ofthe electrode with the skin.

According to a further aspect the invention resides in an electrode beltadapted for use in bioelectrical measurements, wherein the belt is ofelongate form having at least four electrodes spaced along the elongatelength of the belt, the belt comprising a plurality of layers whereinthe belt is constructed to provide active shielding.

According to a preferred feature of the invention, the belt comprises acore and shielding components, arranged in a multi-layer structure toprovide active shielding.

According to a preferred feature of the invention, one of the outerlayers comprises a plurality of apertures spaced along the length of thelayers to thereby expose an underlying conducting layer and therebydefine a corresponding plurality of electrodes.

According to a preferred feature of the invention, each electrode has aconductive track connecting each electrode with a termination.

According to a preferred feature of the invention, the belt ismanufactured by a process similar to that used for printed circuit boardmanufacture.

According to a preferred feature of the invention, the belt is adaptedfor use with EIT measurements.

According to a further aspect, the invention resides an apparatus fordetecting changes in tissue volume and its rate of change by EITanalysis comprising:

first processing means;

second processing means; and

electrode means adapted to apply a predetermined current between a pairof electrodes spaced at the surface of a body portion, under control ofthe first and second processing means and also adapted to measure in aplane through the body portion, the voltage between pairs of electrodesat the surface of the body portion resulting from the applied current toprovide a set of voltage measurements, wherein the electrodes extend ina substantially linear orientation across one side only of the bodyportion;the first processing means being operable to receive the set of measuredvoltages and to provide the set of voltage measurements to the secondprocessing means, the first processing means being further operable toreceive sets of voltage measurements at predetermined intervals and toprovide these sets to the second processing means, the second processingmeans being operable to determine the changes in the voltage measurementbetween consecutive sets of voltage measurements; to provide areconstructed image of the body portion; to determine the resistivityindex of the tissue from the reconstructed image; to derive a volume oftissue from the determined resistivity index; and to determine the rateof change of tissue volume between consecutive sets of voltagemeasurements.

Preferably, the first processing means is further operable to measurevoltage noise levels.

Preferably, the second processing means is further operable to initiatean alarm where the rate of change of tissue volume is above apredetermined value.

Preferably, the second processing means is operable to determine theresistivity index by calculating a vector indicative of changes involtage measurements between consecutive sets of voltage measurements;multiplying the vector by a reconstruction matrix, the resultant matrixbeing the reconstructed image.

Preferably, the resistivity index is generated by integrating overpixels in the reconstructed image.

Preferably, the electrode means comprises a belt including amultiplicity of substantially equidistantly spaced electrodes, wherebycurrent can be applied to any pair of the multiplicity of electrodes,and the voltage measured from one or more pairs of the multiplicity ofelectrodes.

Preferably, the first processing means is operable to apply current toall pairs of electrodes on the belt, and to take voltage measurementsfrom all possible pairs of electrodes, for each current electrode pair.

Preferably, the second processing means is provided remote from thefirst processing means.

Preferably, the current is applied to all pairs of electrodes on thebelt, and voltage measurements are measured from all possible pairs ofelectrodes, for each current electrode pair.

Thus, the apparatus and method of the present invention provides asignificant number of advantages over known methods. It detects the rateof bleeding, and is particularly suitable for use with young people. Itis non-invasive, low cost, and can avoid the need for surgery. It issmall, portable and light and easy to use, and so could be used, forexample, at the scene of an accident. In addition, it does notnecessarily require special skills. It is sensitive, and can be usedeven for small amounts of fluid.

The belt has the advantage that it does need to be placed all the wayaround a patient's abdomen, thereby reducing any discomfort to thepatient or risk of aggravating an existing injury, and facilitating itsuse for operators.

The invention will be more fully understood in the light of thefollowing description of several specific embodiments:

BRIEF DESCRIPTION OF THE DRAWINGS

The description is made with reference to the accompanying drawings, ofwhich:

FIG. 1 is a block diagram of the component parts of the apparatus of thepresent invention;

FIGS. 2A to 2G are plan views of the layers that make up the electrodebelt of the apparatus of FIG. 1;

FIG. 3 is an enlarged schematic cross-section through an electrode onthe belt of FIGS. 2A to 2G illustrating the layered structure; and

FIG. 4 is a schematic block diagram of the components of the on-patientmodule 2 of FIG. 1;

FIG. 5 is a diagrammatic representation of the processing flow ofinformation by the processor; and

FIG. 6 illustrates a 16×16 array for use in representing data measuredby the apparatus of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The embodiments of the invention are directed to an EIT system andmethod adapted to detect changes within a body. They are particularlysuited to detecting internal bleeding within the peritoneum.

As previously mentioned, EIT systems apply a current to a body andmeasure voltage between electrodes placed on the surface of the body.From these measurements it has been possible to calculate the electricfield which is created in a two-dimensional plane or three dimensionalvolume as a result of the electric current flow. A variation in electricfield results from changes in resistivity of the various tissues withinthe region. From these resistivity changes, it is possible to create animage of changes in organs and tissue present in the region. While thishas been done in the past, the quality of the images that have resultedhave been quite poor and thus the process has had limited practicalimaging application. The inventors have recognised that while intrinsicimage quality may be poor, the process may be used to provide aparameter calculated from the output of an EIT imaging system which issubject to change and can be monitored in real-time. Internal bleedingis a particular phenomenon which causes the specified parameter to varydue to the significantly different resistivity of free blood incomparison with that of other tissue in the abdomen. Pooling of blood(or other conductive fluids) create a localised anomalous electricalconductivity which perturbs current flow within the body, and thereforeperturb impedance measurements made on the body surface. Theseperturbations can be measured via the imaging process, to enablecalculation of the rate of change, volume and location of the anomalousfluid. In this way the inventors have identified a manner in which EITcan be used to identify and monitor internal bleeding by non-invasivemeans.

As the most common cause of intraperitoneal bleeding is blunt traumareceived in motor vehicle accidents the inventors have been concerned todevelop apparatus that may be used for use with such patients who maywell be suffering other injuries, including spinal injuries and/or beunconscious. EIT equipment of the prior art is often not suitable foruse in such circumstances.

The first embodiment is described with reference to FIGS. 1 to 6. Asshown in FIG. 1, the first embodiment is an EIT system 1 adapted todetect internal bleeding in the peritoneum and comprises a flexibleelectrode belt 3, an on-patient module 2 and a processing means 4.

As better shown in FIGS. 2A to 2G, the electrode belt 3 is of elongateform having a plurality of electrodes 7 equally spaced along its length.Unlike the belts of the prior art which are adapted to be placed aroundthe perimeter of the abdomen, the belt of the embodiment is arranged tobe placed against the anterior surface of a patient's abdomen, so thatit runs substantially from one side of the patient's abdomen to theother side. It is thus considerably shorter than conventional electrodebelts which have been used for EIT. In typical use, the belt 3 is placedproximate the umbilicus. The belt has some flexibility to enable it toadapt to the contour of the abdomen and hold the contact faces of allelectrodes in firm contact with the skin of the abdomen to therebyensure satisfactory electrical conduction. In use, the contact faces arepre-gelled with an appropriate conductive paste to assist contact withthe skin, and contact is further assured by means of adhesivesurrounding each electrode face. It can be seen that the belt thusdefine a substantially linear array of electrodes across the abdomen, asrequired for EIT analysis. The electrode belt 3 is used to apply acurrent between a selected pair of the electrodes and to makemeasurements of voltages between remaining pairs of electrodes.

The contact faces of the electrodes are elongate and preferably ofsubstantially rectangular form. The elongate axes of the contact facesare oriented transversely to the elongate direction of the belt, so thatwhen the belt is applied to the abdomen the contact faces liesubstantially parallel to a central axis of the abdomen. Elongateelectrodes have been found to produce a more uniform parametersensitivity in images. Internal bleeding can be present anywhere withina large region of the abdomen, and, in use, the belt 3 is usuallyapplied centrally in the region of the umbilicus. The elongate shape ofthe contact faces of the electrodes ensures that the electric fieldapplied to the abdomen is relatively uniform over a wide region of theabdomen and that, consequently, the rates estimated and gathered fromthe abdomen are relatively insensitive to their axial location relativeto the electrodes. The length of the contact faces is selected as acompromise to provided extended length while ensuring good contact withthe body. Typically, a length in the range of 75 mm to 100 mm has beenfound to be optimum, although lengths outside these limits will stillfunction. The width is selected to ensure good contact area, whileproviding adequate spacing between electrodes. Typically, a width in therange of 5 mm to 25 mm has been found to be suitable.

The contact faces are composed of silver/silver chloride or any othersuitable electrode material. Each electrode 7 has a conductive track 8connecting the electrode 7 to an electrical termination 9.

While the EIT system of the invention can be adapted to function withelectrode belts constructed in the manner of the prior art, it is verydesirable to minimise electrical noise. Because the measurement ofvoltages between a pair of electrodes is intrinsically a high impedancemeasurement, when the voltage signals are processed, they aresusceptible to noise. Thus, noise suppression or insulation must beused. In the present embodiment, active shielding is provided in which acoaxial configuration is used by means of the novel construction of thebelt. In this way, signals are transmitted from the belt to theon-patient module 2, with the signal being applied both to the core andto the shield of the coaxial configuration. By doing this, there is nocapacitive coupling between the shield and the core, because there is nodifferential voltage between the core and the shield. A low impedanceshield voltage is generated using follower amplifiers in the on-patientmodule 2.

The electrode belt of the embodiment is constructed in a manner similarto a flexible printed circuit. In the present embodiment, the coaxialconfiguration is created using a seven-layer arrangement—as isillustrated in FIGS. 2A to 2G. FIGS. 2A to 2G each show one of the sevenlayers.

The two outer layers 10 a, 10 b comprise an insulating material and one10 a includes apertures 12 to allow the electrodes 7 to contact thepatient's skin. There is also provided an aperture 11 for a termination9.

As mentioned above, each electrode 7 comprises a core and shieldarrangement. FIG. 2C illustrates the core layer 10 c. The core layercomprises the electrodes 7, each electrode having a printed conductivestrip 8, which will be used to connecting the electrode 7 to the coaxialcable 5. Each conductive strip 8 leads to a termination 9, which in turnconnects to the coaxial cable 5. The conductive strips 8 are also madefrom a suitable conductive material such as silver. On either side ofthe core layer 10 c are insulating layers 10 d and 10 e, which provide alayer of insulation between the core and the shielding. It can be seenfrom FIGS. 2A to 2G that one insulating layer 10 e provides insulationover the electrodes 7, while the other layer 10 d does not, therebyallowing the electrodes 7 to protrude through the apertures 12 on theouter layer 10 b. On either outer side of the insulating layers 10 d and10 e are the shielding layers 10 f and 10 g. These layers 10 f, 10 ginclude shielding for the electrodes 7 and conducting strips 8 of thecore layer 10 c, and are also made of a conductive material such assilver.

The two outer layers 10 a, 10 b are then placed on the outside. The belt3 is printed using conventional printing techniques, with alternatelayers of silver and insulating material. The traces of the differentlayers have different widths. The shielding layers 10 f, 10 g have thesame, and thickest, width and are printed in silver. The insulatinglayers 10 e, 10 d have a smaller width and are printed in insulatingmaterial. The core layer 10 c has the smallest width and is also printedin silver. If the shielding layer 10 f is printed first, then subsequentlayers laid down will take on the shape indicated in FIG. 4, thusforming a complete shielding layer around the core. FIG. 4 shows, asmentioned above, the detail of a single electrode 7, and illustratesthis.

As mentioned above, the core and shielding conductive strips areterminated with a suitable termination 9 including a core portion and ashield portion that can then be coupled to a coaxial cable 5. Thetermination 9 comprises sixteen separate terminations—a core andshielding component for each electrode 7.

In this way, signals can be sent to and from the electrodes 7 from theon-patient module 2, to carry out the appropriate measurements as willbe described in more detail below.

The on-patient module 2 comprises processing circuitry and a telemetrytransceiver 21 that will allow data to be transferred to and from theprocessor 4. In the embodiment, the data is transmitted by wirelesscommunication to remove the need for a physical connecting cable betweenthe body under test and the processor. Nevertheless, it should beappreciated that a wired connection could be used as an alternative.

The components of the on-patient module 2 are illustrated schematicallyin FIG. 4. These components are standard components and are mounted, ina conventional, known manner on a printed circuit board (PCB) (notshown). The on-patient module 2 applies current to a selected pair ofadjacent electrodes 7, and reads voltages from other pairs of adjacentelectrodes 7. Current is supplied via a current multiplexer 13 and aconstant current source in response to signals from a direct digitalsynthesiser (DDS) 14, and digital signal processor (DSP) 16. As anexample, the DSP 16 can be an Analog Devices ADSP-2181 and the DDS 14can be an Analog Devices AD9850. The actual current is provided by acurrent source 23 provided between the DDS 14 and the currentmultiplexer 13. A current monitor 15 measures the actual current appliedto the electrodes 7—which may be slightly different to the constantcurrent selected to be applied to the electrodes 7, due to the sourceresistance of the body—and transmits the measured value of the currentto the DSP 16, to ensure that the correct current value of the currentis communicated to the processor 4. The DDS 14 controls the frequency ofthe current signal to be applied to the current multiplexer 13. In thepresent invention, the current is usually selected at around 3 mA and 62kHz, although this can be varied.

When current is applied to a pair of adjacent electrodes, then theresulting voltages between other pairs of electrodes on the electrodebelt 3 are measured These voltages are input via a voltage multiplexer17 to a differential amplifier 18 to provide a voltage difference, dV,which is then input to an analogue to digital converter (ADC) 24, whichthen provides a 14-bit data signal to the DSP 16 corresponding to thisvoltage difference.

Controlling signals and data can be sent to and from the DSP 16 to theremote processor 4 via a serial communications port 20. As mentionedabove, this data is sent using radio telemetry, and a suitable radiotelemetry transceiver 21 is provided on the on-patient module 2.However, it will be understood that other communications means—eitherwireless of fixed line—could be used. The on-patient module 2 alsoincludes a battery (not shown), which supplies power to the components,as well as to the electrode belt 3.

The software for the on-patient module 2 resides on an EPROM 22. Uponreset of the DSP 16, it boots from the EPROM memory.

The software consists of a main routine and approximately 20 subroutinescarrying out various functions. The main routine carries out theinitialisation of the various circuit elements on the PCB and thenenters an infinite loop waiting for events, to which it responds. Eventsare initiated by the receipt of characters on the serial port 20 of aUART board coupled to the DSP 16 and memory mapped into the DSP dataarea. The arrival of particular character strings causes selectedactivities to be executed within the software subroutines. Severalinterrupts are enabled for the DSP 16. A timer interrupt is used tostart and stop activities that need to be done in a timely fashion. Thetransmission and reception of characters on the UART connected to theDSP is also done using interrupts.

Character strings sent to the UART 20 from the processor 4 are used toinvoke the following activities:

-   -   Test whether the On-Patient Module 2 is on and communicating        properly.    -   Select two electrodes on the electrode belt 3, to which the        current is to be supplied.    -   Select electrodes on the electrode belt 3, from which voltages        are to be measured—this is usually all other possible pairs of        electrodes on the belt, that is apart from the electrodes to        which the current is supplied.    -   Select the gain of the differential amplifier 18 used to amplify        the voltage measured on the selected voltage electrodes.    -   Select the frequency for transmission of the signal to the        electrodes i.e. that of the current applied to the belt 3. Four        frequencies are available—15625 Hz, 31250 Hz, 62500 Hz and        125000 Hz. The default is 62500 Hz.    -   Carry out a single measurement of current (using the current        monitor 15), and the voltage using the presently selected        current and voltage electrode pairs from the belt electrodes 7.    -   Carry out a complete measurement of all possible voltage and        current readings from all possible current electrode pairs on        the electrode belt 3. In one instantiation, eight current source        positions and forty voltage measurements are made in total.        Electrode pairs include the two end electrodes between which        measurements are taken/current is applied.    -   Stop all measurements, calculations and activities being        undertaken.    -   Measure On-Patient Module 12 battery health.

Prior to the initiation of the above functions—by the receipt of acharacter string by the UART serial port 20 connected to the DSP 16—theDSP 16 transmits a string back out the serial port 20 to the processor4, to verify that the command was received. The DSP 16 carries out allthe logic to convert the bit stream arriving at its serial port intomeaningful characters. Characters sent through the UART serial port 20are mapped into the memory of the DSP 16. The clocking signal of serialport 1 is used to control the triggering of the ADC 24. The timerinterrupt is set up to allow timer interrupts to be used to start andstop data gathering. Interrupts for the reception and transmission ofdata on the serial port of the UART serial port 20 are enabled. Allextraneous serial port interrupts are cleared and nesting of interruptsis disabled. Programmable flag pins are set to be outputs rather thaninputs.

The DDS 14 operates in a conventional, known manner.

The current monitor 15 includes a programmable gain amplifier thatamplifies the signal to the current monitor 15. The differentialamplifier 19 amplifies either one of the battery signal or the voltagefrom the selected voltage electrodes. Both amplifiers have their gainsset to one of 4 values. The gain of the current monitor PGA may be setto 1, 10, 100 or 1000. In practice it is set to 1000 because the currentmonitor signal is small. The gain of the other PGA is set to 1 (forbattery signal) or 10, 100 or 1000 (for voltage measurement). Twocontrol lines are required for both amplifiers to program one of thefour gains. These control lines are connected to programmable flagoutputs on the DSP 16 and thus gains are set by the DSP 16. Aprogrammable switch is used to select what signals are sent to thesecond PGA above—that is either the voltages from electrodes 7 orbattery voltage and subsequently to the ADC 24.

There are four 8-channel multiplexers on the PCB. Two are in the currentmultiplexer 13 and two are in the voltage multiplexer 17. Themultiplexers select one of the eight electrodes 7 to be the positivecurrent electrode, and a second to be the negative current electrodei.e. each selects one of the pair of electrodes to which the current isapplied; and the positive voltage electrode and the negative voltageelectrode i.e. the pair of electrodes between which a voltagemeasurement is taken. The multiplexers are set by logic levels suppliedby two 8-output programmable latches connected to the DSP 16. One latchis programmed to output the four logic levels (3 inputs to set thechannel and 1 to enable) required for each of the 2 multiplexers in theelectrode current circuit and the other is likewise programmed to outputsettings for the voltage multiplexers. The latches reside on the databus of the DSP 16 and are programmed with a write to the DSP's data bus.

A programmable switch is used to select whether the ADC 24 is suppliedwith a signal from the current monitor 15, or via the differentialamplifier 18. This switch is set by a logic level output from aprogrammable flag on the DSP 16. Clocking of the ADC 24 is carried outby the clock line from serial port 1 on the DSP 16. No other use is madeof serial port 1. Clocking of the ADC 24 is undertaken at a rate of 32times the frequency transmitted through the current electrodes of theelectrode belt 3. The clocking rate is set by writing a counter valueinto a register on the DSP 16.

The output of the ADC 24 is wired to the IDMA port on the DSP 16.

Prior to enabling a voltage or current measurement the IDMA port is setup to start writing into a particular memory location in the DSP'srandom access memory (RAM), thus storing the measured value at theselected memory location. The DSP's IDMA port increments the pointer tothe write location after each analogue to digital conversion.

In one instantiation, 8000 samples of waveforms are recorded. Thiscorresponds to 250 periods of the transmitted waveform with 32measurements per period. The 14 bits from the ADC 24 are written intothe 14 least significant bits of the chosen 16-bit location in the RAM.The second most significant bit is zero. The most significant bit is anoverflow test bit from the ADC 24.

The 8000 samples are then used to provide a measured value for thecurrent and voltage. Thus, after 250 periods are recorded for therequired measurement, that measurement is processed and the resultstransmitted to the processor 4 via the UART serial port 20. A baselinelevel for each measurement is calculated. This is done to allow forvoltage offsets on amplifiers, the ADC 24 and the electrodes 7.

In one instantiation, each of the 32 voltages in the period of thecurrent or voltage waveform is estimated by averaging them over the 250periods. This results in 32 numbers. Once appropriately normalised theyare compared with the full averages over 250 periods to derive astandard deviation measure of the validity of signal measurement.

After subtraction of the baseline level, the amplitude of the AC signalfrom the current monitor or voltage electrodes is calculated by summingthe square of the 32 averaged samples.

For calculation of the battery voltage (a DC signal), a simplenormalised average of the 8000 samples is calculated.

Data and standard deviations are formatted as 32-bit real floating-pointnumbers and transmitted out the UART serial port 20 to the processor 4.

The processing of the data by the processor 4 is representeddiagrammatically by FIG. 5. The processor 4 may take any suitable formsuch as a handheld computer or personal digital assistant, for example aHewlett Packard Jornada or iPAQ palm size computer or any serial capabledevice. In the embodiment, the processor is associated with a version ofthe Windows CE operating system, although it will be recognized thatother suitable operating systems may be used.

The analysis of the data is directed to the calculation of a parameterwhich is representative of conductivity within the portion of the bodybeing monitored. It is the change of this parameter which provides anindication in increase in volume of body tissue as in the case ofinternal bleeding. This parameter can be monitored over time todetermine the rate of internal bleeding. This parameter has been termedthe “Resistivity Index” (RI).

In order to process the information, the abdomen may be modelled as adisk-shaped region. This is represented in FIG. 6. As shown in FIG. 6,the disk-shaped region 60 is represented by a 16×16 array 61. A 16×16array is used on the basis that 16 electrodes would be placedsubstantially equidistantly around the circumference i.e. abdomen, withthe applied electric field patterns resulting in values for electricalconductivity in the surrounding tissue at each of 256 array locations.This is typical of EIT processing that has been conducted in the priorart. However in principle there is no restriction on the number ofpixels used in an image. The array may be represented as a planarsurface in two dimensions as shown in FIG. 6, or may be mapped as athree dimensional cylindrical array of voxels.

The Resistivity Index involves adding up the total conductivity changeobserved within an image since this total change should reflect thetotal volume of anomaly that has appeared. In the case of thepixellation shown in FIG. 6, calculating the Resistivity Index will justinvolve adding all pixel values since all pixels are the same area, butin general should allow for variations in pixel size and therefore ingeneral the Resistivity Index should be calculated by summing thequantity (dσdS) where S is the region described in the model, e.g. acylinder.

The equation defining this relationship may thus be expressed as shownbelow for a two-dimensional array:

RI = ∫_(Ω) d σ d Sor for a three-dimensional array:

RI = ∫_(Ω) d σ d V

Alternatively, these may be expressed in discrete format. For atwo-dimensional array, the relationship below is given, where A_(p) isthe area of the pixel in question and TP is the total number of pixels:

${RI} = {\sum\limits_{p = 1}^{TP}\;{d\;\sigma\; d\; A_{p}}}$

For a three-dimensional array, the relationship below is given, whereV_(p) is the volume of the voxel in question and TP is the total numberof voxels:

${RI} = {\sum\limits_{p = 1}^{TP}\;{d\;\sigma\; d\; V_{p}}}$

As shown in FIG. 5, the processor 4 receives data from the on-patientmodule 2 which is operated on at 42 with the stored reconstructionmatrix 43 to provide a reconstructed image 44. The image per se may bedisplayed on the display 53 of the processor. This image is thencombined with stored spatial filter matrix 45 to provide a filteredimage 47. A Resistivity index is then calculated at 49. Preferably, theResistivity Index is processed with a temporal filter 50 at 51 to removebreathing effects. Finally a scaling is applied to derive a blood volumeestimation. This process is now described in more detail below.

The processor 4 therefore receives information as to the voltagesmeasured between respective electrode pairs for a given measured currentbetween a predetermined pair of electrodes. Measurements are receivedfor all possible electrode pairs. In an instantiation using 8 adjacentcurrent electrode pair positions and adjacent voltage measurements thiscomprises a total of 40 measurements—that is for each electrode pair towhich current is applied, there are 5 voltage measurements to be taken.There are 8 different electrode pairs (including the two endelectrodes)—making the total number of measurements 40 i.e. 8×5. Inaddition, the actual current value measured between each of the eightadjacent current pairs is measured and transmitted, making a total of 48measurements sent to the processor 4.

These 40 voltage measurements, normalised by dividing by each relevantcurrent measurement, are saved to file in memory in the processor 4.

Routines concerned with data collection are:

Routine Name Function select_measurement(int type); Select Voltage,current or battery check setup_current(int cPair); Requests particularcurrent pair be used setup_shorted_current(int cPair); Sets up shortedcurrent source (used for measuring offsets in channels) setup_volts(intvPair); Requests particular voltage pair be used get_data(double *mean,Collect a particular data value double *sigma); get_volt_dataget_curr_data get_battery_data DataOnly(int nsamp); Collect complete setof data without CSubject class DataPresent( ); Ping

Other routines include:

BatteryVoltsOK( ); Entire routine to check on module battery voltage

And routines concerned with Higher Level Data collection include:

CollectShortedData( ); Collect a complete set of shorted dataCollectData(CSubject *patient); Collect complete set of dataBreathingCycleOk(double *respIndex); Function to filter out breathingcycle effects in measurements Calibrate(BOOL *pflag, int *gflag,Calibrate module CSubject *patient, CVIndex *vValues); BeltContactOk(intcontactNo); Checks that RMS noise on differential voltage measurementsis low. In this routine, for each adjacent voltage pair, thedifferential voltage is measured. If the standard deviation in themeasurement is above a threshold, then it is determined that the contactof the belt to the patient is bad.)

Processing these 40 measurements will give an indication of theconductivity of the tissue within the abdomen at the time that themeasurement is taken. By repeating these measurements regularly atpredetermined intervals, any changes in the measured conductivityindicates changes in the tissue composition e.g. through the presence ofinternal bleeding, as well as the approximate location of that change.

Once all the measurements have been saved to a file associated with theprocessor 4, then the processor 4 is operable to carry out theReconstruct routine mentioned above, to determine the rate of change ofblood volume in the abdominal cavity, and, if necessary, trigger anyalarm.

Routines concerned with data reconstruction include:

Routine Name Function LoadBMatrix(wchar_t *filename); Reads inreconstruction matrix from file void RemoveOffset(CCompleteMeas *data);Removes offset from voltage and/or current dataXferandNormalise(CSubject *mSubject, Normalises voltage data byCCompleteMeas *set); current data, moves it to CSubject data structureProcess(Csubject *exp, Csubject *ref, Calculates vector of changes inCsubject *w); data from reference set. This vector, denoted w, ismultiplied by the reconstruction matrix to produce an image and hence anestimation of blood volume Reconstruct(CSubject *ref, CSubject *exp,Process, reconstruct, estimate RIType *ri, RIType *riblood, BYTE *pixel,RI, embed image in larger BOOL fStatus) background, interpolated_sparsemult(double *colinput, double Performs multiplication of*colresult, int n, int length, SPARSELIST voltage differences by*eltlist); reconstruction matrix Partial(Csubject *b, RIType *ri); SumPixel Values Discretize(double **bhires, int *pixels); Rescalehiresolution image to 0->256 Reconstruct(CSubject *w, CRecon *b)Multiply Processed data by Reconstruction matrix BloodConvert(RIType ri,RIType *riblood); Convert Partial output (of RIType) to blood volumeCalculateRate(RIType *rate, RIType *blood); Estimate rate in terms ofblood quantity by dividing the quantity determined by output of Partialby time interval between this data and reference data collection time.int CheckAlarm(RIType *rate); Classify rate calculated below by severity

In the present embodiment, eight electrodes are provided over half theabdomen—numbered “0” to “7”—and this must be accounted for, and whichwill be discussed further below.

By solving Laplace's equation for the region of interest (for theabdomen the region shape will usually be assumed cylindrical), theexpected resultant electric field from current electrodes placed atpositions around the circumference of the region 60 can be determinedand can be used to derive expected conductivity values measured at anyof the 256 locations within the array 61, for current electrode pairsplaced around the circumference. Thus, a 256×256 reconstruction matrixcan be derived from these calculations for all possible electrode pairs.This reconstruction matrix will be used by the processor 4 to provideindications of changes in tissue conductivity within the abdomen—as willbe discussed in more detail below.

This Reconstruct routine comprises a number of sub routines.

A flow chart for this reconstruct routine is as follows:

-   1. Process—that is calculate the difference between reference (ref)    and current (exp) data sets to obtain vector, w    -   1.1. Reconstruct—this comprises multiplying the data vector w        (256×1 matrix) by a reconstruction matrix (256×256) to obtain a        reconstructed image.-   2. If necessary, reduce spatial variation of image parameters by    filtering, and apply temporal filtering to remove breathing    artefacts-   3. Integrate the pixels in (256×1 or 16×16) reconstructed image to    obtain an RI estimate-   4. Scale rate estimate using empirical sensitivity to obtain RI in    terms of blood volume-   5. Divide estimated blood volume by time interval between reference    and current data sets-   6. Depending on the rate that has been calculated, determine alarm    category:

Thus, the process subroutine calculates the change between the presentmeasurements and the last set of measurements—referred to herein as“exp” and “ref” respectively. Thus the change=(exp−ref). This is done bycalculating a vector, w, of changes in data from the reference (“ref”)data, which can later be multiplied by the reconstruction matrix toproduce an image and hence an estimation of blood volume—as will bediscussed in more detail below.

The resultant 256×1 matrix is an approximate reconstructed image, and,from there, values for the “resistivity index” (RI) can be obtained byintegrating pixel values in this reconstructed image.

This RI value can then be used to provide an estimate of blood volume.This is done by using empirically derived values of blood volume as afunction of RI. The value of estimated blood volume can be used todetermine the rate of change of blood value by dividing the estimatedblood volume by the time interval between the reference (ref) andcurrent (exp) data sets, and if this value falls greater than apredetermined value, then an alarm can be triggered.

As mentioned above, variations to the measurements that can beattributed to the patient's breathing can be accounted for within theprocessing—if required.

The elongate shape of the electrodes 7, on the electrode belt 3, enablecorrelation between the reconstructed image and the amount of tissuethat the image represents. Elongate electrodes overcome the problem thatthe use of conventional-shaped (small diameter, circular) electrodeswould present, in that, if an amount of tissue such as blood, were tomove a small axial distance out of the plane of the electrodes then theresistivity index would be very different. Providing elongateelectrodes—as described above—overcomes this problem to some extent andallows the vector/reconstructed image to be used to derive theinformation required.

The processor can be used to select to a variety of parameters foroperation and function. For example, the following functions areaccessible using the processor 4:

-   -   Starts automatic collection at specified time intervals    -   Stop automatic measurements    -   Changes time interval between measurements    -   Checks RMS noise appearing on adjacent electrode voltage        measurements to determine contact quality    -   Change the bleeding rate displayed between /sec, /min or /hr    -   Executes a measurement    -   Restarts measurements (change patient)    -   Saves data from a session    -   Checks for communication between the processor 4 and on-patient        module 2    -   Setup communication (serial port) between the processor 4 and        on-patient module 2    -   Checks Battery of module 2    -   Changes measurement frequency (at present measurements are made        at 62.5 kHz).    -   Change phase of measurement e.g. to take quadrature (reactive)        measurements rather than resistive measurements.

It will be obvious to person skilled in the art, that variations arepossible within the scope of the present invention. For example, theapparatus could be used to detect other fluids or other tissue—such ascancerous tissue—and in other areas of the human body, and could beadapted for use with animals.

It will be appreciated that advances in technology may lead to otherways of implementing certain aspects of the embodiments. Those skilledin the art will appreciate that the wireless communication may beimplemented in other ways than that described.

In an adaptation of the embodiment, the belt is provided with somestiffness to hold the belt in curved form, having greater curvatureproximate the ends. Such a belt is adapted to support electrodes veryclose to the sides of patient, maintaining those electrodes in goodcontact with the skin. In the present embodiment mechanical contactbetween the skin and electrodes is facilitated by adhesive electrodesurrounds.

It will be clear that the invention is not restricted to a belt havingthe number of electrodes described in the embodiment. With too fewelectrodes, there is insufficient resolution of voltage variationsacross the abdomen, so that is becomes impractical to generate a clearenough reconstruction using this method. For some uses, acceptableresults may be obtained with a belt having only four electrodes,although for most uses, at least 8 electrodes would be preferred. Thenumber of electrodes might also be increased above eight to improveresolution. Clearly, such an array will require more processing powerand data transmission bandwidth to be effective. But the effectivenessof taking this step will be limited in any event. As the number ofelectrodes is increased, the relative resolution improvement reduces sothat the benefit becomes insignificant.

It is also to be appreciated that the purpose of the belt is to providea straightforward means of applying a group of electrodes to the skin inthe desired area. It would also be possible provide a linear array ofelectrodes which are adapted to contact the skin in an arrangement whichwould not be considered as a belt in conventional terminology. Forinstance, the electrodes might be associated with a mattress such thatcontact with the electrodes might be maintained merely because thepatient was to lie upon the mattress. Such an array would still contactthe abdomen on one side only and require EIT analysis in the same manneras previously described to thereby provide the monitoring for internalbleeding.

Throughout the specification, unless the context requires otherwise, theword “comprise” or variations such as “comprises” or “comprising”, willbe understood to imply the inclusion of a stated integer or group ofintegers but not the exclusion of any other integer or group ofintegers.

We claim:
 1. An EIT system adapted to detect changes in tissue volume ina body portion, the EIT system comprising a belt having a core layer, aninsulating layer on either side of the core layer, a shielding layer oneither side of each of the insulating layer, and an outer layer oneither side of each the shielding layer, wherein the core layercomprises a plurality of spaced apart electrodes to be applied inelectrical contact with skin of the body portion, wherein the insulatinglayers expose only one side of the electrodes, wherein each shieldinglayer comprises a conductive material providing circumferentialshielding for the belt wherein one of the outer layers includes aplurality of apertures corresponding to said electrodes to provideengagement of only one side of the electrodes on the skin of the bodyportion: a current source adapted to cyclically apply an electriccurrent between one pair of the electrodes, a voltage measuring means tomeasure the voltage across each of the other pairs of the electrodesresulting from the current, a data collection system and a data analysissystem to analyse data resulting from the voltages that are measured bythe voltage measuring means, wherein the analysis system is configuredto obtain quantitative information related to amounts and rates ofconductive tissue changes occurring in the body, based on an EITanalysis equivalent to that obtained from data derived from electrodesspaced around the full perimeter of the body portion.
 2. An EIT systemas claimed in claim 1 wherein the data analysis system establishes amodel of the body portion under analysis comprising a plurality ofelements and wherein a parameter representative of an electric fieldpresent in each element resulting from the current is calculated fromthe voltages that are measured and wherein the values of at least aportion of the parameters that are calculated for the elements areamended to substantially reconstruct values that would be obtained frommeasurements of voltages around the perimeter of the body portion andwherein the change of value of the parameter in a portion of elementsover time is indicative of change in tissue volume within the bodyportion.
 3. An EIT system as claimed in claim 2 wherein the dataanalysis system implements a series of steps to reconstruct theparameter values of the elements, the steps comprising: calculating thedifference between a reference data set and a measured data set of thevoltages as measured to establish a vector; multiplying the data set bya reconstruction matrix to obtain a reconstructed image having aplurality of pixels; integrating the values of the pixels in thereconstructed image to obtain a value of the parameter; and monitoringchange in the value of the parameter over a period of time to provide anindication of change in tissue volume.
 4. An EIT system as claimed inclaim 1 wherein a detected change in tissue volume is representative ofinternal bleeding.
 5. An EIT system as claimed in claim 4 wherein theparameter is defined as Resistivity Index calculated in accordance withone of:${RI} = {{\int_{\Omega}^{\;}{d\;\sigma\ d\; S\mspace{14mu}{or}\mspace{14mu}{RI}}} = {\sum\limits_{p = 1}^{TP}\;{d\;\sigma\; d\; A_{p}}}}$for a two-dimensional array, or${RI} = {{\int_{\Omega}^{\;}{d\;\sigma\ d\; V\mspace{14mu}{or}\mspace{14mu}{RI}}} = {\sum\limits_{p = 1}^{TP}\;{d\;\sigma\; d\; V_{p}}}}$for a three-dimensional array.
 6. An EIT system as claimed in claim 5wherein the data analysis system further implements the steps of: usingempirical sensitivity calibration to provide an estimate of theparameter in terms of blood volume; dividing the estimated blood volumeby time interval between reference and measured data sets to provide anestimate of the rate of bleeding; applying a spatial filter to produce aresistivity index that is independent of bleeding location within imagedregion applying a temporal filter that reduces the effect on theresistivity index of shape and conductivity changes resulting from thesubject's breathing cycle; and determining an alarm category dependingon the rate of bleeding that has been calculated.
 7. An EIT system as inclaim 1 wherein the data analysis system applies a digital filter to thedata to provide temporal filtering of the data to thereby remove or atleast minimise the effect of breathing on the EIT analysis.
 8. An EITsystem as in claim 1 wherein the current source, voltage measuring meansand data collection system are associated with an on-patient moduleadapted to be carried by the body having the body portion, wherein thedata analysis system is provided by a remote processor and wherein datacommunication is provided between the on-patient module and the remoteprocessor.
 9. An EIT system as claimed in claim 8 wherein the datacommunication is by wireless communication.
 10. An EIT system as claimedin claim 8 wherein the on-patient data module comprises processingcircuitry and a telemetry transceiver that will allow data to betransferred to and from the processor.
 11. An EIT system as claimed inclaim 10 wherein the processing circuitry selects the pair of electrodesto which a current is applied and the pair of electrodes across whichvoltage is measured at any point in time.